Allocated fluxes%buoy in MOM_ice_shelf

This change avoids [this](https://github.com/NOAA-GFDL/MOM6/blob/dev/master/src/parameterizations/vertical/MOM_entrain_diffusive.F90#L277) fatal error when using pure layer mode without defining BULKMIXEDLAYER.

src/ice_shelf/MOM_ice_shelf.F90

@@ -1391,6 +1391,9 @@ subroutine initialize_ice_shelf(param_file, ocn_grid, Time, CS, diag, fluxes, Ti
        !GM, the following is necessary to make ENERGETICS_SFC_PBL work
        call allocate_forcing_type(G, fluxes, ustar=.true., shelf=.true., &
                                   press=.true., water=.true., heat=.true.)
+       ! This is to make it work with layer mode without BULKMIXEDLAYER 
+       allocate( fluxes%buoy(isd:ied,jsd:jed) )
+       fluxes%buoy(:,:) = 0.0
   else
     if (is_root_pe())  print *,"allocating fluxes in solo mode"
     call allocate_forcing_type(G, fluxes, ustar=.true., shelf=.true., press=.true.)